Interference suppressing circuit



2 sheets-sheet 1 Filed Dee.

68 MWD... @52% k blk l0 T z, 3 OUTPUT 20 Feb. 23, 194s. H. B1 RUBI'N2,311,696

INTERFERENCE SUPPREssING CIRCUIT Filed Dec.l 5', 1941 2 Sheets-Sheet 2Patented Feb. 23, 1943 entete UNETED STATES PATENT OFMCE {NTERFERENCESUPPRESSING CIRCUIT Hyman B. Rubin, ew Haven, Conn.

Application December 3, 1941, Serial No. 421,515

5 Claims. (Cl. Z50-20) 'I cuit elements which I employ, behave quitedif- My invention relates to a circuit and means for suppressinginterfering voltages, such as aperiodic disturbances on received radiosignals, Whether such interference be from natural or man-made sources.Circuits for this purpose are often called static eliminators. Mycircuit is adapted for the reception of voice or code or tone or picturemodulated signals.

In the past, many devices and arrangements have been tried for thispurpose but all have been found to have serious drawbacks. A commonfault of such arrangements is that the method employed for reducing thestrength of the interfering voltage also greatly reduces the strength ofthe desired signal voltage. These dilculties are avoided in my circuit.

In my circuit I separate the interfering voltage component from both thecarrier-frequency and the audio-frequency components of a derivedportion of the desired signal, and apply the separated interferingvoltage in phase opposition to the interference component of the totalreceived voltage, thus erasing the interfering voltage component appliedto the conventional receiving circuit, whose output delivers the desired signal substantially free from interference. My circuit isentirely responsive to small values, as well as large values, of ratioof static to signal.

An object of my invention is to provide an interference eliminator whichwill substantially eliminate all interfering voltage components withoutmaterially reducing the desired signal component voltage.

Another object of my invention is to provide an interference eliminatorwhich separates the interference voltage component from the signal.

modulation and the carrier-frequency component of a derived portion ofthe total received signal, and applies the interference voltagecomponent in phase opposition to the total received voltage signal whichis being transmitted through the receiver. thus cancelling outsubstantially all of the interference component.

Still another object of my invention is to provide a static eliminatorwhich employs multigrid tubes to apply dii-ferent voltage components ofthe total received signal in phase opposition to balance out certaincomponents.

'I'he interference voltage wave is not of closely dened frequencycharacteristics but is substantially aperiodic, while the regulardesired signal modulation and the carrier frequency and side bands havewell dened frequency characteristics and hence after passing throughtuned cirferently than do the interference Yvoltage compcnents of thewave.

'Ihe interference voltage component has a high decrement, which makespossible the ready separation, by suitable discriminating circuitelements, of the interference component from the desired portions of thesignal.

I find that the interference elimination cir-l cuits which I providework, if anything, better withv very sensitive receivers, than with arelatively insensitive receiver, whereas the opposite condition usuallyobtains with static eliminators heretofore known.

Y With the Aabove and other objects in View? which will more readilyappear as the nature of the invention is better understood, the sameconsists in the novel construction, combination and arrangements ofparts hereinafter more fully described, illustrated and claimed.

In the accompanying drawings, wherein like characters of referencedenote corresponding parts in the different views:

Figure 1 is a block diagram showing sche- .V matically the basicoperative yrelations between the different units of my circuit, and

' I may employ is known as the 6SK7 which is a applying a voltagerepresenting the same com,

ponent in phase opposition to another grid 'ofv the same tube. Onestandard type of tube which super-control tube. Another type of standardtube which I may employ with certain increased advantages is the GSA?,Both these tubes are pentodes. The 6SA7 has the advantage that one gridis tied to the shell of the tube structure and hence is at groundpotential, but this tube has the disadvantage that it has a much greatertendency to oscillate, since it was originally designed as anoscillator. In the operation of my circuit, as a static eliminator, I donot desire that these tubes oscillate.

I derive a portion of the total voltage of the incoming signal,including carrier frequency and desired modulation, and interferencevoltage components, separate out two adjacent side bands, and thenrectify the side bands, separate the rectified side bands respectivelyinto the modulation component and carrier frequency component, feed backas bias the rectified modulation component in phase opposition to themodulation component of the unrectified side band, and apply therectified carrier-frequency component to the unrectiiied side band inphase opposition to its carrier-frequency component. The resultantoutput of` the tubes after these feed-back operations have beenconsummated is simply the interference voltage component, which.

is adjusted to be in opposite phase to the interference component of thetotal received signal passing through the receiver, and is applied tothe received signal to cancel out substantially al1 of the interferencecomponent and deliver a clear understandable signal with the desiredtransmitted modulation.

Figure 1 shows the different paths traversed by the derived portion ofthe total received signal, its separation into side bands, the separateyrectification of the side bands, their respective separation intocarrier-frequency and modulation components, and the feed-back paths ofeach of these components in phase opposition to the respective componentof the total received signal.

Referring particularly to Fig. 2, a source of modulated radio-frequencysignal is shown at 2 and is applied through suitable conventional meansto the signal grid of a receiving tube I, which is shown as a multi-gridtube. For lpurposes of illustration the receiving circuit here describedmay be considered to have tube I constitute an intermediate stage of asuper-heterodyne. The input signal from 2 may be applied through theprimarywinding 4a of an input transformer 4 whose secondary 4b isshunted by variable condenser l which is connected to sig-nal grid II oftube I. The secondary 4b of transformer 4 is tuned to the carrierfrequency of the desired incoming signal. Tube I has cathode I4,suppressor grid I2, -plate I3, oscillator grid 9 and shield grid I0, andthis cathode I4 is connected to ground through resistor I6- shunted bycondenser I5. Oscillator grid 9 is connected to ground through resistorI1 which is conveniently about 20,000 to 50,000 ohms. Oscillator grid 9is connected to suppressor grid I2 through condenser I9 which isconveniently 0.1 microfarad. The output of tube I is connected to theprimary of transformer 20 which is tuned by shunted condenser 2I, andthe secondary of transformer 20 is connected to the output terminals 3of theusual main signal line of a superheterodyne: receiver.

A wire 23 in which variable condenser'5 is'in'- serted in series, isconnected to input terminal 2-of the total received signal, and isapplied to one terminal of `primary winding 25 of transformer 24 whichis shunted by variable condenser 28. The other terminal of primary 25may be connected as shown to the low terminal of primary winding 4a. Thecircuit including primary 4a and primary 26 may be tuned by'variablecondenser 5 and is tuned to the carrier frequency of the desiredincoming signal. While I have shown one condenser 5 for tuning bothprimary 4a and primary 26, it is sometimes Vpreferable to use twoseparate variable condensers for tuning these two primaries separately.Condenser 8 is conveniently about 0.1 microfarad. Primary 4a and primary25 are not mutually inductively coupled. Transformer 24 is provided withtwo secondary windings 26 and 21 which are respectively tuned byvariable condensers 29 and 30. Transformer 24 may advantageously have aoneto-one winding ratio.

The primary winding 25 together with primary winding 4a is tuned byvariable condenser 5 to the carrier frequency of the desired incomingsignal, which for purposes of illustration may be taken as 450kilocycles. Secondary winding 26 would in this case be tuned to includeor peak at a mean value of an upper representative side band frequency alittle above the main carrier frequency, such as 5 kilocyclesthereabove, which in the case taken for illustration would be 455kilocycles. Secondary winding 21 is tuned by variable condenser 30 toinclude or peak at a mean value of a lower side-band frequency a littlebelow the main carrier frequency, such as 5 kilocycles therebelow, whichin the case taken for illustration would be 445 kilocycles. In this waythe circuits connected to transformer 24 do not constitute a substantialdrain on the main received desired signals of the principal carrierfrequency, but the side bands areV respectively transmitted through thetwo secondary windings together with side band modulation components, asWell as the aperiodic interference. The .part of the derived portion ofthe signal, which is taken up by secondary winding 26 and is deliveredfrom the tuned circuit constituted by secondary winding 26 and condenser29 is delivered by wire 3| to signal grid 36 of the tube 32 which isdesirably a pentode of the type heretofore described. Tube 32 has acathode 33 and anoscillator grid 34 which, however, in my circuit is notused asv an oscillator. Tube 32 also has suppressor grid 31 and shieldgrid 35, and plate 38. Oscillator grid 34 is connected. to groundthrough resistor 39. Cathode 33 is connected to ground through resistor33a shunted by condenser 33D. Suppressor grid 31 is connected to ground.

The output circuit from plate 38 of tube 32 is connected through wire 53to one primary winding of a transformer 54 which will be hereafterdescribed. Plate 38 is also connected through wire 53 and wire 68 andcondenser 69 to an anode 13 of a double diode 10. Instead of doublediode 10 there may be employed two single diodes. To anode 13 of diode10 corresponds cathode 1I. Double diode 10 also has another anode 14 towhich corresponds cathode 12. The output of anode 13 is connected toground through resistor 15 andresistor; 16 in series, constitutinganoutput circuit. A tap 11 is provided on resistor 16 to provide propernegative bias on signal grid 3E with respect to cathode, and signal grid46 is similarly biased. Resistor 16 is about 0.1. to l megohm. The.variable tap 11 is connected, as shown, to one terminal of secondarywinding 26 of: transformer 24, so that any rectified output from diode:anode 13 which isv delivered to wire 3| and signal grid 36 passesthrough secondary winding 26 in series, which will not pass carrierfrequency from a voltage thus applied, and only the modulation getsthrough. This results in continuously applying to signal grid 36 of tube32 a varying bias which varies in magnitude exactly in accordance withthe modulation of the received signal. Because of the inherentrelationships between output and input of tube 32, and by adjustment ofthe circuit elements, this varying bias is exactly out of phase with themodulation of the received unrectied modulated radio-frequency wavewhich is applied to signal grid 36 from primarywinding 25, and in thismanner I` find that the resultant instantaneous voltage on signal grid36 cancels out the modulation vfrom the output of tube 32. I nd,however, that the interference voltages of relatively high decrement donot thus cancel themselves out from the output of tube 32.

From the common connection of resistor and resistor 16, the wire 8Ileads to resistor 82 shunted by condenser 83. Resistor 82 may, in actualpractice, have a resistance from about 0.1 megohm to 1.0 megohm.Condenser 83 may in practice have a value of about 100 micromicrofarads.This arrangement of resistor 82 shunted by the condenser 83 blocks themodulation component of the output from anode 13, but permits theradio-frequency component to pass through to connecting wire 84 and maybe considered to act as a phase corrector and timer. The arrangementconstituted by resistor 82 and condenser 83 is adjusted to provideproper phase relation in the radio-frequency output which is deliveredby wire 84 to oscillator grid 44 of tube 42.

'Ihe electron stream passing from cathode 43 of tube 42, to its plate48, is thus subjected to a radio-frequency voltage which corresponds tothe carrier frequency but is adjusted to be exactly out of phasetherewith, so that no carrier frequency appears in the output deliveredfrom plate 48 of tube 42 through output winding 56. Oscillator grid 44is connected to ground through resistor 49 which is conveniently about20,000 to 50,000 ohms, as is also resistor 39. tube 42 is connected toground. Cathode 43 of tube 42 is connected through resistor 43a shuntedby condenser 43h to ground. Resistor 43a. as well as 33a areconveniently 500 to 1,000 ohms. Condenser 43h, as well as 33h, isconveniently about .A

0.1 microfarad. It will be seen from Fig. 2 that the circuit throughwhich the. diverted signal passes is symmetrical about the center oftransformers 24 and 54 and diode 10, and that there arecross-connections between certain portions of,

each of the symmetrical halves of the circuit.

Thus, the other'anode 14 of double diode 10 is connected throughresistors 18 and 19 in series to ground, and resistor 19 is providedwith variable tap 80 which is connected to one end of secondary winding21 of transformer 24, whereby only the modulation component is appliedthrough wire 4I to signal grid 46 of tube 42. Likewise wire 85 leadingfrom the common point of resistors 18 and 19 delivers through resistor86 shunted by condenser 81 and through wire 88 only thecarrier-frequency component to oscillator grid 34 of tube 32.

Screen grid 45 is by-passed to ground and also connected to B+ potentialthrough resistor 50 which is conveniently about 20,000 ohms. Screen gridis similarly connected through resistor 30. Screen grid 45 is alsoconnected to suppressor grid 41 through condenser 5| which is conventiently about 0.1 microfarad. Suppressor grid 31 is similarly connected.In this manner Athe electron stream passing from cathode 33 to 'anode 38of tube 32 is subjected to a modulated radio-frequency voltage appliedfrom primary 25 which includes the interference voltage, and is alsosubjected to the out-of-phase rectified modulation bias componentdelivered from wire 11 through secondary 26 and wire 3l, and also to theoutof-phase rectied carrier-frequency component delivered through wire88 and resistor 86 and condenser 81 to oscillator grid 34 of tube 32.'I'his causes the erasing or cancellation in tube 32 of both the carrierfrequency side band component and the desired signal modulation periodiccom- Suppressor grid 41 of A ponent of the'total received signal,whereby the 75 net resultant output from anode 38 of tube 32 asdelivered through wire 53 to one of the input windings 55 of transformer54, consists entirely of the interference voltage. I

The one input winding 55 of transformer 54 is adjusted in impedance tomatch the high output impedance of tube 32. Input Winding 55 is tuned bycondenser 58 Vto the side-band frequency through wire 3l, Input winding56 is tuned by condenser 59 to the side-band frequency through wire 4|.The other input winding 56 of transformer 54 is similarly adjusted inimpedance to match the high output impedance of tube 42.

One end of input Winding 55 is connected to ground through condenser 6I, conveniently about 0.1 microfarad, and is also connected to vpositiveplate battery through resistor 62. One endof input winding 56 isconnected to ground through condenser 64, which is conveniently about0.1 microfarad, `and is also connected to positive plate battery throughresistor 63. Resistors 62', 63, are conveniently 1,000 to 5,000 ohms.

Transformer 54 in actual practice advantageously has a one-to-onewinding ratio. The phase relations of the voltages applied to inputwindings 55 and 56 are suitably adjusted to apply to output winding 51of transformer 54, a combined voltage, which as previously explainedconsists almost entirely oi the interference voltage which is beingapplied to input terminal 2. Output winding 51 is here lshown as shuntedby Variable condenser 60, and may be tuned in desired manner, andconveniently is tuned to the carrier frequency of the desired signal. Itis, however, possible to get good results while omit-"- ting condenser60 vand not tuning winding 51.

YBy adjustments including phasing of output Winding 51 with respect towindings 55 and 55, the voltageV on wire 22, as applied to grid 9 oftube I, is out of phase with the voltage on signal grid II of tube I.Since, as has been explained, the output energy from output winding 51through wire 22 consists almost entirely of the interference voltage,the net result is that the output of tube I delivered from this anode I3has erased therefrom substantially all of the interference voltagecomponent, and contains only the main carrier frequency with its desiredsignal modulation. Transformer winding 26 of transformer 24, as abovestated, has a resonance curve which in cludes vor peaks at a mean valueof an upper representative side-band frequency a little above the maincarrier frequency such as 5 kilocycles thereabove. Similarly secondarywinding 21 of transformer 24 is tuned by variable condenser 30 toinclude or peak at a meanv value of a lower side-band frequency a littlebelow the main car-` rier frequency suchv as 5 kilocycles therebelow. Inoperation, the interference eliminator is connected to the inputterminal 2 by applying the connecting wire 23 thereto, and eliminatortuning condensers 5, 29, 30, are adjusted in the manner above describedto respectively tune to the main carrier which may be the intermediatefrequency and the upper and lower side bands. Ordinarily, in asuperheterodyne receiver the interference eliminator circuit will bepermanently connected and the variable tuning elements will bepermanently tuned to the inter-v mediate frequency and its side bandsrespectively. The necessary adjustments are then made in the circuitelements as described to adjust for phase oppositionof thecOmpOnentswhich are to be Abalanced-out. In-this manner the output of the tube lVhas substantially all of the interference component removed therefrom,so that the output of output transformer 20 as delivered to outputterminal 3 consists substantially entirely of the desired modulatedsignal.

My invention is of great importance and utility in the reception of allknids of intelligence modulated radio-frequency signals, and has beenvery successfully employed for eliminating interference in reception ofbroadcast, telegraph, highfrequency, and video modulated radio-frequencysignals.

While I have described my invention as applied for purposes ofillustration to the intermediate stage of a superheterodyne receiver, itwill be obvious to those skilled in the art that my invention issusceptible of application to substantially any kind of radio receivingcircuit With equally good results, and also that the details which Ihave shown and the particular form of tube which I have illustrated inthe auxiliary balancing circuit which I have specifically described, aresusceptible of various modifications, without in any Way departing fromthe disclosure or the spirit of my invention, and all such modificationsthereof which are comprehended within the scope of the appended claims Iconsider to be included Within my invention.

I claim:

l. The method of eliminating undesired interfering voltage componentsfrom a total received low-frequency modulated radio-frequency signalwhich consists in deriving a portion of the total incoming signal,rectifying a part of the so derived portion, separating thelow-frequency signal modulation from the unmodulated carrier frequencyof the so rectied part, and separately feeding back and separatelyapplying said signal modulation and said carrier frequency of saidrectified part to said unrectied derived portion in phase opposition tothe signal modulation and carrier-frequency components thereofrespectively, and applying the residual interfering voltage component ofsaid derived portion in phase opposition to said total received signal.

2. The method of eliminating undesired interfering voltage componentsfrom a total received llow-frequency modulated radio-frequency signal,

which consists in deriving a portion of the total incoming signal,selecting two symmetrically closely spaced side bands thereof,rectifying each of said side bands separately, separating thelowfrequency signal modulation from the carrier frequency of each ofsaid rectified side bands, separately feeding back and applying therectified low-frequency signal modulation of each said side band to theunrectied same said side band in phase opposition to the low-frequencysignal component thereof, separately feeding back and applying the sorectified carrier-frequency component of each said side band to theunrectied other said side band in phase opposition to thecarrier-frequency component thereof, and applying the residual unrectiedinterfering voltage component of said derived portion in phaseopposition to said total received signal.

3. In radio reception employing an auxiliary lateral circuit comprisinga pair of multi-grid control tubes, the method of eliminating undesiredinterfering voltage components from a total received low-frequencymodulated radio-frequency signal which `consists in deriving a portionof the total incoming signal, selecting two symmetrically closely spacedside bands thereof,

amplifying each of said side bands by one of said control tubesrespectively, rectifying each of said so amplied side bands separately,separating the low-frequency signal modulation from the carrierfrequency of each of said rectified side bands, separately feeding backand applying the rectified low-frequency signal modulation of each saidside band to one grid of the said tube which ampliies the unrectied samesaid side band in phase opposition to the low-frequency signal componentthereof, separately feeding back and applying the so rectifiedcarrier-frequency ccmponent of each said side band to a different gridof the other of said tubes which amplifies the other said side band inphase opposition to the carrier frequency component thereof, andapplying the residual unrectifed interfering voltage component of saidderived portion as delivered from the outputs of said tubes in phaseopposition to said total received signal.

4. In an interference eliminator, a source of incoming receivedmodulated radio-frequency total signal containing interference voltagecomponents, a radio-receiving circuit for receiving said signal, anauxiliary circuit for adjusting the components of said received signaland comprising a pair of translating means, the input of said auxiliarycircuit being connected to said source for deriving a portion of saidreceived signal, means in said auxiliary circuit for selecting a pair ofspaced side bands from said derived portion and applying themrespectively to each of said translating means, rectifying meansconnected for rectifying the output of each said translating means,means for separating the modulation component from the carrier frequencycomponent of the output of each of said rectifying means respectively,means for feeding back and applying the modulation component of theoutput of each said rectifying means to the input of the same saidtranslating means which is connected thereto in phase opposition to themodulation component of the unrectied side band delivered thereto, meansfor feeding back and applying the so separated carrier frequencycomponent of the output of each said rectifying means to the input ofthe other said translating means which is not connected thereto in phaseopposition to the carrier frequency component of the unrectied side banddelivered thereto, and means for combining the outputs of saidtranslating means containing interference components and applying thesame to said receiving circuit in phase opposition to the interferencevoltage component therein.

5. In an interference eliminator, a source of incoming receivedmodulated radio-frequency tota-l signal containing interference voltagecomponents, a radioreceiving circuit for receiving said signal, anauxiliary circuit for adjusting the components of said received signaland comprising a pair of multi-grid tubes, the input of said auxiliarycircuit being connected to said source for deriving a portion of saidreceived signal, a pan` of adjustable reactance means in said auxiliarycircuit for selecting a pair of spaced side bands from said derivedportion and applying the same to a first grid of each of said tubesrespectively, a pair of rectifying means connected for rectifying theoutput delivered from the plate of each of said tubes, the output ofeach of said rectifying means being respectively connected to the one ofsaid reactance means through which the portion of the signal therebyrectied had passed before rectification, said reactance means throughwhich the portion of the signal thereby rected did not pass beforerectiiication, said -connecting means being respectively adapted toadjust the phase of the carrier frequency rectiiied component passingtherethrough, and output means for combining the outputs of said tubescontaining interference components and applying the same to saidreceiving circuit, said output means comprising phase adjusting meansfor adjusting the phase of said interference component.

HYMAN B. RUBIN.

